I used a high density foam core board for the tunnel entrance - scribed with a pencil - coated in white glued and then sprinkled with sand for texture. Once dry paint grey and dry brush with a lighter grey and white.

I probably made the tunnel entrances bigger than they need to be... I didn't want the cars hitting them though

Been a while since your last post on this one I thought it was a dead duck, The bridge is coming along very nicely indeed.
Is the radius of the curve inside it likely to cause a problem with cars coming off or, can you hoon without issue or, will the momentum of a derailed car just conveniently slide out.
In any case I look forward to more updates.

I'm with you there, I much prefer driving a car than racing one, it is nice to see a car fishtailing around corners and not have someone saying "you know fishtailing will slow you down" I know but it looks good and is fun.

Have you put any of your cars up that you have built I don't recall seeing any.